This invention relates to neutron-neutron logging and more particularly to a borehole logging system for use in simultaneously determining both the porosity of the formation surrounding the borehole and the macroscopic absorption cross section of the formation surrounding the borehole.
In such neutron-neutron logging, a fast neutron source irradiates the formations surrounding the borehole. The resulting secondary radiation is measured by at least one thermal neutron detector axially spaced from such source within the borehole. In U.S. Pat. No. 3,491,238 entitled POROSITY LOGGING EMPLOYING TWO THERMAL NEUTRON DETECTORS SPACED RELATIVELY FAR FROM NEUTRON SOURCE, issued Jan. 20, 1970, to Linus S. Allen, there is disclosed a neutron-neutron logging system in which two thermal neutron detectors are spaced apart at distances from a steady-state neutron source where the ratio of the thermal neutron flux measured by the two detectors is influenced more by epithermal neutron parameters than thermal neutron parameters. The ratio of the thermal neutron flux detected at such distances is therefore a measure predominantly of a single epithermal neutron parameter of the formations, i.e., the slowing down length, which is indicative of porosity.